Tons of pitchers pick up their pivot foot so that both feet are off the rubber before placing the pivot foot down at the front edge of the rubber.
Being a foot to the forward, I'd say it's illegal.
If it went to the same side along with the left foot then slid back into position in front of the rubber, then it might look strange, but I don't see anything that would be more illegal or deceptive than any other pitcher who lifts and drops the pivot foot. It's only a matter of degree.
I would like to see a video because I'm sure each of us is visualizing this differently in our heads.